Effect of pre-oxidation on high temperature sulfidation behavior of FeCr and FeCrAl alloys

High temperature corrosion of structural alloys in sulfur bearing environments is many orders of magnitude higher than in oxidizing environments. Efforts to increase sulfidation resistance of these alloys include addition of alloying elements. Aluminum additions to iron-chromium alloys bring about increase in sulfidation resistance. This paper reports the effect of pre-oxidation on the sulfidation behavior of Fe-20Cr and Fe-20Cr-5Al alloys in H2-2% H2S environment at 800 °C. The surfaces of sulfidized specimens were also examined. Pre-oxidation of the two alloys results in an incubation period during subsequent sulfidation. After this incubation period, the Fe-20Cr alloy showed sulfidation behavior similar to that when the alloy was not pre-oxidized. The incubation period during sulfidation of the Fe-20Cr-5Al alloy was significantly longer, over 45 h, compared to 2 h for the Al free alloy. Based on the microscopic and gravimetric data a mechanism for sulfidation of these alloys with pre-oxidation has been proposed

Effect of surface deposited rare earth oxide gel characteristics on cyclic oxidation behavior of Fe20-Cr alloys

Rare earths have been used to increase high temperature oxidation resistance of many chromium dioxide and alumina forming alloys. These rare earths can be added as elements (or as oxide dispersions) to the alloys or applied as an oxide coating to the alloy surface. The sol-gel technique is considered to be very efficient to apply fine oxide particle coatings. Oxide gel coatings of various rare earths such as lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, yttrium, erbium and ytterbium have been applied to an iron-chromium alloy to determine their influence on the cyclic oxidation behavior (RT-900 °C) of the alloy. The morphology and coverage of the rare earth oxide gels varied with the type of rare earth. The cyclic oxidation resistance of the alloy increased with increase in time at temperature required to reach a specific chromium dioxide layer thickness and this in turn was influenced by the rare earth ion radius and characteristics of the rare earth oxide coating such as morphology, stability, coverage, resistance to thermal stresses and consequently adhesion

Cyclic oxidation resistance of rare earth oxide gel coated Fe-20Cr alloys

Rare earths (RE) have been used to increase high temperature oxidation resistance of chromium dioxide and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as an oxide coating to the alloy surface. RE oxide gels, obtained by the sol-gel technique, were used to coat Fe-20Cr specimens prior to cyclic oxidation tests in the temperature range RT-900 °C. The cyclic oxidation resistance of the alloy increased with time required to reach a specific chromium dioxide layer thickness, and this was influenced by RE ion radius, the coating morphology and coverage. Extended cyclic oxidation tests from peak temperatures of 900, 1000 and 1100 °C at cooling rates of 330 and 1000 °C/s revealed the marked influence of La2O3 coatings. The role of RE in increasing overall oxidation resistance of chromium dioxide forming alloys is discussed